1. Field of the Invention
The present invention relates to the field of integrated circuit memory; and more particularly high density read-only memory ROM devices.
2. Description of Related Art
ROM devices include an array of ROM cell transistors which permanently store data, such as computer programs, in electronic devices. ROM devices are preferred over other types memory because they provide high density, low cost storage for data which does not need to be changed during the operation of the system incorporating the ROM. One high density read-only memory architecture is described in U.S. Pat. No. 5,117,389 entitled FLAT CELL READ-ONLY INTEGRATED CIRCUIT invented by Yiu, issued May 26, 1992. Other example architectures are described in Millman, MICROELECTRONICS, McGraw-Hill (New York, 1979), Section 9-3, pages 276-283.
The density of ROM cells on an integrated circuit is directly related to the cost of storing data. Thus, the manufacturing techniques for ROM devices have developed to provide higher and higher density architectures. One primary approach to achieving higher density is improvements in manufacturing processes which allow for smaller line widths.
An alternative technique for increasing the density of ROM devices has been proposed in U.S. Pat. No. 5,383,149 entitled "ULSI Mask ROM Structure and Method of Manufacture," invented by Hong, issued Jan. 17, 1995. In the Hong patent, a double-density device is provided by stacking two arrays of ROM cells, with one array above and with one array below a shared wordline array. The bottom array of ROM cells is formed using n-channel transistors, while the top array of ROM cells is formed using p-channel transistors in the Hong design. This use of different conductivity types in the transistors has a number of deficiencies. First, the wordlines must drive different polarities of wordline signals depending on whether the p-channel or n-channel transistor arrays are being sensed. Second, an asymmetry in the design occurs because p-channel transistors are slower than n-channel transistors. Therefore, the n-channel transistor array has a faster access time than the p-channel transistor array, slowing the overall speed of the device. Furthermore, the mask ROM code implant steps used in the two types of array require different materials and processing steps. This increases the complexity, the number of steps and the cost of the manufacturing process.
Accordingly, it is an object of the present invention to provide a high density ROM device, overcoming the disadvantages of the Hong approach, and providing even greater density.